1. Technical Field
The presently disclosed technologies are directed to a system and method for improving image quality by adjusting the printing frequency. The system and method described herein measure the belt surface velocity and use it to adjust the printing frequency.
2. Brief Discussion of Related Art
In order to ensure good image quality (IQ) in cut sheet direct to paper (DTP) ink jet printing, a media transport system must transport the media through the print zone at a controlled rate. Methods of choice for the media transport use a belt module that tacks the media to the surface of the belt using vacuum or electrostatic forces. In order to assure accurate printing, the belt with the tacked media must be held firmly against a support. Deviations of belt surface velocity from a nominal value cause defects in the printed image. These deviations can be caused by, but are not limited to, velocity variations caused by belt thickness variations, variation in drag over the belt support area and roll tolerances in the belt module.
FIG. 1 shows an exemplary production printing system that could be used with the present invention. The media is tacked to a belt using electrostatic or vacuum forces and the media transport belt module transports the sheet through a printing zone. In addition, the belt is held firmly against a support (e.g., a flat platen as shown in FIG. 1 or a curved surface). Deviations in the belt transport velocity can cause color registration error, i.e., color separations so that colors are not printed at the intended locations. This results in image quality defects.
Reflex printing is a method known in the art, which is used to mitigate image quality defects. Conventional reflex printing is done on a belt module roll or, in the case of a drum architecture system, the printing is done on the main drum encoder. In reflex printing, the frequency of the image generation is a function of a measured velocity of the printing media passing under the print heads. In its simplest implementation, the image generation frequency (i.e., the firing of the print heads) is proportional to the measured roll/drum angular velocity. This allows two or more colors printed by print heads in different locations to coincide and provides a great improvement in image quality.
In a belt module, the angular velocity of a roll is not an extremely accurate measure of the belt surface or media transport velocity. Inaccuracies in the rolls of the module, belt thickness variation, variable drag/friction, etc. contribute to media transport velocity variations that are uncompensated when reflex printing using based on the angular velocity of the belt drive roll. Color-to-color registration errors are positional errors, i.e. they result from the integral of velocity errors over time. For long wave lengths, small velocity errors can integrate to substantial position errors. In contrast, short wave length (very high frequency) errors integrate to a relatively smaller positional error. The belt length represents a long wave length and belt thickness variations are known to cause substantial color-to-color registrations errors. Accordingly, there is a need for a reflex printing system that that can accurately register two or more colors and improve the image quality.